Add HEAD support for storage API object endpoint (#759)

* Support HEAD method on storage API

Problem: As reporeted in #667. The storage API is replying 404 for
existing objects when method HEAD is used, although GET replies with
200.

Solution: Update storage API method route and handler to behave as
"donwload" when HEAD method is used.

* Update documentation regarding flag usage

Problem: As a new user, one tends to simply use what comes in the
README.md file without realizing there is plenty more features at hand.

Solution: Update README.md to pointing out there is a -help flag that
lists all the available configuration overrides. I was tempted to copy
and paste its output, but it might add too much noise for the reader
so disregarded that thought.

Author: jossemarGT

README.md

@@ -84,6 +84,16 @@ It is possible to use fake-gcs-server with signed URLs, although with a few cave
 - You need to configure fake-gcs-server to accept this local URL (by setting
   `-public-host`)
 
+### Available server flags
+
+fake-gcs-server supports various features that can be configured through flags
+upon start. Use the `-help` flag to list all of them alongside their usage
+instructions
+
+```shell
+docker run --rm fsouza/fake-gcs-server -help
+```
+
 ## Client library examples
 
 For examples using SDK from multiple languages, check out the

fakestorage/object.go

@@ -481,7 +481,7 @@ func (s *Server) listObjects(r *http.Request) jsonResponse {
 }
 
 func (s *Server) getObject(w http.ResponseWriter, r *http.Request) {
-	if alt := r.URL.Query().Get("alt"); alt == "media" {
+	if alt := r.URL.Query().Get("alt"); alt == "media" || r.Method == http.MethodHead {
 		s.downloadObject(w, r)
 		return
 	}

fakestorage/server.go

@@ -231,7 +231,7 @@ func (s *Server) buildMuxer() {
 		r.Path("/b/{bucketName}/o/{objectName:.+}/acl").Methods(http.MethodGet).HandlerFunc(jsonToHTTPHandler(s.listObjectACL))
 		r.Path("/b/{bucketName}/o/{objectName:.+}/acl").Methods(http.MethodPost).HandlerFunc(jsonToHTTPHandler(s.setObjectACL))
 		r.Path("/b/{bucketName}/o/{objectName:.+}/acl/{entity}").Methods(http.MethodPut).HandlerFunc(jsonToHTTPHandler(s.setObjectACL))
-		r.Path("/b/{bucketName}/o/{objectName:.+}").Methods(http.MethodGet).HandlerFunc(s.getObject)
+		r.Path("/b/{bucketName}/o/{objectName:.+}").Methods(http.MethodGet, http.MethodHead).HandlerFunc(s.getObject)
 		r.Path("/b/{bucketName}/o/{objectName:.+}").Methods(http.MethodDelete).HandlerFunc(jsonToHTTPHandler(s.deleteObject))
 		r.Path("/b/{sourceBucket}/o/{sourceObject:.+}/copyTo/b/{destinationBucket}/o/{destinationObject:.+}").Methods(http.MethodPost).HandlerFunc(jsonToHTTPHandler(s.rewriteObject))
 		r.Path("/b/{sourceBucket}/o/{sourceObject:.+}/rewriteTo/b/{destinationBucket}/o/{destinationObject:.+}").Methods(http.MethodPost).HandlerFunc(jsonToHTTPHandler(s.rewriteObject))

fakestorage/server_test.go

@@ -290,6 +290,13 @@ func testDownloadObject(t *testing.T, server *Server) {
 			map[string]string{"accept-ranges": "bytes", "content-length": "9"},
 			"something",
 		},
+		{
+			"HEAD: using storage api",
+			http.MethodHead,
+			"://storage.googleapis.com/storage/v1/b/some-bucket/o/files/txt/text-01.txt",
+			map[string]string{"accept-ranges": "bytes", "content-length": "9"},
+			"",
+		},
 		{
 			"HEAD: bucket in the path",
 			http.MethodHead,